The present invention relates to a monitoring and a compensation of the angle of the rotor in an electric machine with an angle sensor that determines the actual angle of the rotor.
An electric machine, in particular a synchronous machine, usually includes an angle sensor by which the position of the rotor with respect to the stator can be determined. The angle sensor transmits the present angle (actual angle) of the rotor to an inverter that provides alternating-current signals, for example three-phase current signals, to the windings of the stator depending on the present angle of the rotor. The efficiency of the electric machine can be increased through adjusting the three-phase current signals on the basis of the angle of the rotor with magnets. The way in which an electric machine operates is known to one of skill in the art, and need not have to be repeated in the interests of conciseness.
As a result of manufacturing tolerances, there is an angular offset between the position of the rotor measured by the angle sensor and the true position of the rotor. At present, the angular offset is determined by way of an expensive apparatus after the electric machine has been manufactured. The angular offset can be stored as a correction value, and taken into account in the output of the three-phase current signals. The angular offset can change during the service life of the electric machine. The angular offset, once determined, can be deleted as a result of software problems or an incorrect operation during service. The mounting of the angle sensor can change as a result of vibration, temperature or aging effects. It can, moreover, be necessary to exchange the angle sensor in a dispatched vehicle.
DE 10 2012 017 794 A1 discloses a drive with a synchronous machine in which an angle sensor is not used and the position of the rotor is determined by means of an injection device.
DE 10 2010 016 105 A1 discloses a pitch-drive apparatus capable of emergency operation for a wind turbine or hydroelectric power plant with a synchronous motor that comprises an angle sensor. In an emergency operation, the speed of rotation of the motor can be determined on the basis of measured injection currents.
In an electric machine for the drive of a motor vehicle, the efficiency is reduced and a torque with a lower precision is output if the offset of the angle sensor from the true position of the rotor is not corrected. To increase the efficiency of the electric machine, it is necessary to compensate the angle of the rotor with the angle sensor with the greatest long-term stability possible.
The invention addresses the object of providing a method and an apparatus for determining the offset of an angle sensor with respect to the rotor of an electric machine.
The object of the invention is achieved by a method and apparatus in accordance with embodiments of the invention.
The method according to the invention for monitoring an electric machine of an electrically driven vehicle comprises the step of determining a first value of a position of a rotor by way of an angle sensor when the rotor is located in a first position, and determining a second value of the position of the rotor by way of an injection method when the rotor is located in the first position. The method further comprises the step of determining a first difference between the first value and the second value of the position of the rotor for the first position of the rotor. According to the invention, a check is made as to whether the first difference falls below a predetermined threshold value.
The angle sensor can be used to determine the position of the rotor during the operation of the electric machine. The present angular position of the rotor can in this way be determined in a simple manner. The injection method can be used to determine the offset between the true angular position of the rotor and the angular position of the rotor determined by the angle sensor. It is obvious that this offset can be taken into account in the controller, for example in the case of an inverter, to generate three-phase current signals that take the true angular position of the rotor better into account, in order to produce a higher efficiency and a torque with a higher precision.
The injection method essentially injects a signal into the d-axis and measures the current response in the q-axis. The injection method is known to those of skill in the art, and therefore for the sake of conciseness does not have to be described in more detail here. An exemplary implementation of the injection method is described in DE 10 2012 017 794 A1.
If the first difference falls below a predetermined threshold value, the electric machine is classified as fit for operation. The first difference can be the angular offset. The method classifies the electric machine as suitable for installation into a vehicle if the first difference falls below the predetermined threshold value.
The present invention can be employed both after the manufacture of the electric machine as well as during ongoing operation of the electric machine. The present invention has the advantage that the offset between the value of the rotary position of the rotor determined by the angle sensor and the true rotary position of the rotor can be determined simply and quickly, and without additional test devices.
In a further embodiment, the method further comprises the step of turning the rotor from the first position through a predetermined angle into a second position. The rotor can be turned by means of currents that flow from the inverter into the windings of the electric machine. The method determines a first value of the position of the rotor by way of the angle sensor when the rotor is located in the second position. The method further determines a second value of the position of the rotor by way of the injection method when the rotor is located in the second position. The method further determines a second difference between the first value and the second value of the position of the rotor at the second position of the rotor. The first value and at least one second value can be linked by statistical methods in order to eliminate inaccuracies of measurement, for example inaccuracies of measurement resulting from harmonics.
In one form of embodiment of the method, a mean value is formed from the first and the at least one second difference. A check is further made as to whether the mean value of the first and the second difference falls below the predetermined threshold value. Measurement errors, resulting for example from harmonics that are present in the response signal that is generated by the injection method, can be eliminated through the formation of a mean value. Harmonics in the measurement result of the injection method can be eliminated through this step.
The method can compensate the harmonics of the response to a signal that is applied through the injection method to the windings of the electric machine. The predetermined angle through which the rotor is turned from the first position into the second position can be specified in such a way that it corresponds to half the period duration of a harmonic. Preferably, the predetermined angle corresponds to half the period duration of the harmonic with the greatest amplitude.
The predetermined angle φ can be determined by means of the following formula:φ=360°/(n×2);where n indicates the n-th harmonic, and where n is a natural number.
The harmonic identified with n can be the harmonic with the greatest amplitude.
The method can carry out the step of the determination of the first value of the position of the rotor by way of the angle sensor at a first position of the rotor at the beginning and/or during a journey of the vehicle, and can carry out the step of the determination of the second value of the position of the rotor at the first position of the rotor by way of the injection method. The method can carry out the step of determination of the first difference between the first value and the second value of the position of the rotor at the first position of the rotor at the beginning and/or during the journey of the vehicle. The method can then carry out the step of checking whether the first difference falls below a predetermined threshold value at the beginning and/or during a journey of the vehicle. The method can further carry out the step of storing the first difference in a difference memory in which a plurality of values of the first difference are stored at the beginning and/or during a journey of the vehicle. Through this the method can continuously monitor and adjust the offset between the rotary position of the rotor acquired by the angle sensor and the rotary position of the rotor determined by the injection method. It is assumed in the present invention that the true angle of the rotor can be verified by means of the injection method, since mechanical tolerances between the rotor and the angle sensor have no influence on the result of the measurement by means of the injection method. As a result, the angular offset between the position of the rotor measured with the sensor and the true position of the rotor can be compensated.
The method can average the values of the first differences that are stored in the first difference memory in order to obtain a mean value of the first difference. The method can provide a correction value for correcting the first value of the position of the rotor determined by the angle sensor on the basis of the mean value of the first difference. The correction value can be added to or subtracted from the first values determined by the angle sensor.
The method can check whether proper angle signals are present from all the angle sensors of a redundant angle sensor system. The redundant angle sensors can, for example, comprise a GMR sensor (giant magnetoresistive sensor) and/or an AMR sensor (anisotropic magnetoresistive sensor). If it is determined that only one proper angle signal is present from an angle sensor of the redundant angle sensor system, the method determines whether the first difference between the first value determined by the angle sensor with the proper angle signal and the second value determined by way of the injection method falls below a predetermined value. It follows that the first value determined by the angle sensor with the proper angle signal is located in a tolerance range around the second value determined by the injection method. If it is determined that the first difference falls below a predetermined value, the method can permit an emergency operation of the electric machine. To control the electric machine it follows that only the values of whichever angle sensor of the redundant angle sensors that essentially correspond to the true rotary position of the rotor are used. The values of the respective other angle sensor of the redundant angle sensors are not considered further. The availability of the electric machine can be increased in this way. The method can determine which of the angle sensors of the redundant angle sensor system is providing a proper angle signal.
The invention also relates to a monitoring device that is designed to monitor the operation of an electric machine. The monitoring device includes a determination device that is designed to determine a first value of a position of a rotor by way of an angle sensor at a first position of the rotor, to determine a second value of the position of the rotor by way of an injection method at the first position of the rotor, and to determine a first difference between the first value and the second value of the position of the rotor at the first position of the rotor. The monitoring device further includes a checking device that is designed to check whether the first difference falls below a predetermined threshold value.
The monitoring device can be developed in the way that was previously described in respect of the method.
The invention also relates to an electric drive with an electric machine and the monitoring device. The invention also relates to a vehicle with the electric drive.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.